The electron beam striking a sample in an electron microscope produces x-rays that are characteristic of the material of the sample that is impacted by the electron beam. Consequently, x-ray spectroscopes have been mounted to electron microscopes to analyze the x-rays emanating from the sample. X-rays at wavelengths characteristic of the sample are also produced by fluorescence from interaction of an x-ray beam with the sample, such as in x-ray microscopes. In energy dispersive spectroscopy (EDS), a solid state detector is positioned relatively close to the sample to collect x-rays emanating from the sample. The EDS detector receives and must detect x-rays of many wavelengths, and the resolution of the EDS system is limited by the resolution capability of the available solid state detectors. In wavelength dispersive spectroscopy (WDS), the x-rays emanating from the sample are reflected from a wavelength dispersive element, typically a crystal or multi-layer diffracting element, which reflects the various wavelengths at specific angles. By changing the orientation of the diffracting element or of the position of the detector or both, the wavelength of x-rays that are incident upon the detector after redirection by the diffracting element can be selected, allowing relatively high precision spectroscopy with a capability of resolving relatively narrow peaks. Commonly, the diffracting element has a concave surface so that the x-rays diffracted by the element are either collimated or focussed after reflection onto the detector. The beam spot on the sample, the diffracting element, and detector must all lie on a circle (the Rowland circle) as they are scanned to search new angles--and wavelengths. Thus, a fairly complex and bulky mechanical mounting assembly has been required. The WDS diffractor is thus typically mounted far from the sample in the electron microscope (typically 10 times farther than the detector in an EDS system) and, as a result, collects a relatively small solid angle of the x-rays emanating from the target. Consequently, the photon flux incident on the detector in WDS systems is typically much lower than in EDS systems, and the data collection times in WDS systems are significantly longer than in EDS systems. Generally, a microscope must be run at high beam current to obtain WDS spectra, and in some systems, such as field emission SEM, the beam currents are limited so that conventional WDS cannot be used.
U.S. Pat. No. 5,682,415 to David B. O'Hara describes a conical grazing incidence mirror collimator for x-ray spectroscopy, which can be positioned near a specimen in an electron microscope to receive a relatively large solid angle of x-rays emanating from the sample and to collimate these x-rays by total external reflection into a collimated beam that can be directed to an EDS detector or to a WDS diffractor. By using such a collimator in WDS systems, the diffracting element can be located relatively far away from the sample, preferably outside the electron microscope, with as great as and generally significantly greater photon flux obtained than can be obtained with WDS systems in which the diffractor is mounted within the microscope. In addition, because the beam incident upon the diffracting element is collimated, the diffracting element can be flat rather than curved--as is typically required in prior WDS systems. This is a significant advantage, since a flat diffractor is more efficient at diffracting the entire beam than a curved diffractor. As a result, a WDS system using such collimating optics can be satisfactorily operated at much lower electron microscope beam currents than prior WDS systems.
A limitation of such conical mirror collimating optics is that, while the efficiency of reflection for low energy x-rays (100 eV to 1000 eV) is high, the efficiency of reflection of such conical mirror optics falls off significantly for x-rays at energies above about 1800 eV.